Recently, particulates, such as soot, contained in exhaust gases that are discharged from internal combustion engines of vehicles, such as buses and trucks, and construction machines, have raised serious problems as those particulates are harmful to the environment and the human body. Conventionally, various filters, which are used for collecting particulates in exhaust gases so as to purify the exhaust gases, have been proposed, and filters having a honeycomb structure have also been proposed.
FIG. 4 is a perspective view that shows one type of filter having such a honeycomb structure.
This honeycomb filter 60, which is prepared as a honeycomb structural body made from silicon carbide and the like, has a structure in which a plurality of square-pillar shaped porous ceramic members 70 are combined with one another through sealing material layer 64 that serve as a bonding agent to form a ceramic block 65, and a sealing material layer 63 is also formed on the circumference of this ceramic block 65.
FIG. 5(a) is a perspective view that schematically shows the porous ceramic member constituting the honeycomb filter shown in FIG. 4, and FIG. 5(b) is a cross-sectional view taken along line B-B of the porous ceramic member shown in FIG. 5(a).
The porous ceramic member 70 has a honeycomb structure in which a partition wall 73, which separates a large number of through holes 71 that are placed in parallel with one another in the length direction, serves as a filter.
In other words, as shown in FIG. 5(b), each of the through holes 71, formed in the porous ceramic member 70, is sealed with a plug 72 at either of ends of its exhaust gas inlet side or outlet side so that exhaust gases that have entered one through hole 71 are discharged from another through hole 71 after having always passed through each partition wall 73 that separates the through holes 71.
Here, the sealing material layer 63 formed on the circumference is placed in order to prevent exhaust gases from leaking from the peripheral portion of the ceramic block 65, when the honeycomb filter 60 is installed in an exhaust passage of an internal combustion engine.
When the honeycomb filter 60 having such a structure is placed in the exhaust passage of an internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 73 upon passing through the honeycomb filter 60 so that the exhaust gases are purified.
A filter having such a honeycomb structure, which can collect particulates in exhaust gases, is also designed so that a catalyst used for purifying exhaust gases is adhered to a portion (through holes and the like) functioning as a filter; thus, the filter makes it possible to purify toxic components such as CO, HC and NOx in exhaust gases, to accelerate activation of oxygen, NOx, etc. by the catalyst, and also to reduce activation energy for burning particulates adhered to the catalyst so that the particulates can be burned at low temperatures.
Conventionally, with respect to the filter having the honeycomb structure to which the catalyst is attached, a porous ceramic honeycomb structure formed by refractory particles made from silicon carbide, cordierite or the like has been widely used, and a structural body in which a plurality of porous ceramic members are combined with one another in the length direction through a bonding agent, a structural body which is formed through an extrusion molding process into an integral structure made from ceramics and the like have been generally used (for example, see Patent Document 1).
With respect to the above-mentioned filter using a catalyst, it is preferable to increase reaction sites between the particulates and the catalyst. In order to achieve this structure, it is effective to increase the porosity in a wall portion constituting the honeycomb structural body so that a large number of open pores are included therein; thus, more particulates are collected also in the inner side of the wall portion (hereinafter, referred to as deep-layer filtration) so that the particulates are also made in contact with the catalyst adhered to the inner side of the wall portion.
However, when the above-mentioned methods are used in the porous ceramic honeycomb structural body made from refractory particles, the strength of the filer becomes very low. For this reason, upon burning and removing collected particulates (hereinafter, referred to as a regenerating process) in such a filter, the filter tends to generate a great temperature difference in the length direction of the filter due to the burning process of the particulates, resulting in damages such as cracks in the filter due to the resulting thermal stress. Consequently, the above-mentioned filter tends to lose functions as the filter.
Moreover, with respect to the filter having a honeycomb structure to which a catalyst is applied, a honeycomb structural body manufactured by extrusion-molding a mixture containing inorganic fibers such as alumina and silica, and a honeycomb structural body which is manufactured by corrugating inorganic sheets, which is obtained from inorganic fibers through a paper-making process, have been known (for example, see Patent Documents 2 and 3).
In addition to these, with respect to the filter having a honeycomb structure to which a catalyst is applied, a honeycomb structural body using a metal porous material has also been known (for example, see Patent Documents 4 to 6).
Although the honeycomb structural body using the metal porous material is capable of maintaining sufficient strength even when the porosity is increased, the filtering area becomes very small because of its structure to cause a high flowing rate in exhaust gases upon passing through the filter wall portion and the subsequent high pressure loss in the filter.
Here, with respect to the filter having a honeycomb structure to which a catalyst is applied, a honeycomb structural body in which a plurality of honeycomb ceramic modules, each having a predetermined thickness, are placed with open-hole cells communicating with each other has also been proposed (for example, see Patent Document 7).
Such a honeycomb structural body formed by placing a plurality of the honeycomb ceramic modules makes it possible to alleviate a thermal stress caused by a temperature difference in the filter length direction.
Regarding the honeycomb ceramic modules which forms a honeycomb structural body by placing a plurality of those, the honeycomb ceramic modules formed by extrusion-molding a mixture containing refractory particles and inorganic fibers made from alumina, silica and the like, into a honeycomb structure and then firing the resulting formed body; and the honeycomb ceramic modules formed by perforating inorganic sheets, which are prepared by subjecting inorganic fibers to papermaking process, to form a honeycomb shape have been known.
However, the honeycomb filter constituted by the former honeycomb ceramic modules has no plugged portions, resulting in degradation in the particulate collecting efficiency.
Moreover, the filter having a honeycomb structure is normally used at high temperatures while it is put into a casing (metal container); however, in the case where the former honeycomb structural body formed by placing a plurality of honeycomb ceramic modules is directly put into the casing, since its coefficient of thermal expansion is greatly different from that of the casing (metal container), gaps occur between the modules and the casing (metal container) located on the circumference thereof as well as between the honeycomb ceramic modules; thus, exhaust gases flow out through the gaps, resulting in a leak of collected particulates and the subsequent reduction in the particulate collecting efficiency.
Patent Document 1: JP-A 06-182228 (1994)
Patent Document 2: JP-A 04-2674 (1992)
Patent Document 3: JP-A 2001-252529
Patent Document 4: JP-A 06-257422 (1994)
Patent Document 5: JP-A 06-294313 (1994)
Patent Document 6: JP-A 09-49420 (1997)
Patent Document 7: JP-A 08-12460 (1996)